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<H3><A NAME="SECTION001113300000000000000"> </A>
<A NAME="ch07:hierarchical-keywords"> </A>
<BR>
Hierarchical keywords
</H3>
To avoid possible misinterpretations and naming conflicts for
keywords describing acquisition parameters, ESO has adopted
a hierarchical keyword convention for this purpose using the
keyword <TT>HIERARCH</TT>. Such keywords have the following
syntax:<BLOCKQUOTE>
<TT><TT>HIERARCH</TT> domain level-1 ... level-n keyword = value / comment </TT></BLOCKQUOTE>where the <I>domain</I> always is <TT>ESO</TT>. Several hierarchical
levels may exist with a keyword and associated value at the lowest
level. Naming and parameter follow the general FITS rules when
applicable (<I>e.g.</I> max. 8 characters). Such keywords are mapped
to MIDAS descriptors with names starting with the <I>domain</I>, then
all <I>levels</I> and ending with the <I>keyword</I> where the
components are separated with the full stop character ('.'). The
hierarchical keyword:<BLOCKQUOTE>
<TT><TT>HIERARCH ESO TEL FOCU SCALE = 1.489 / Focus length (deg/m) = 5.36"/mm</TT></TT></BLOCKQUOTE>will yield the MIDAS descriptor name <TT>ESO.TEL.FOCU.SCALE</TT> while<BLOCKQUOTE>
<TT><TT>HIERARCH ESO INS OPTI-3 ID = 'ESO#427 ' / Optical element identifier</TT></TT></BLOCKQUOTE>will become the descriptor <TT>ESO.INS.OPTI-3.ID</TT>. When writing such
descriptors out again, the reverse translation is applied.
<P>
In MIDAS releases prior to <TT>96NOV</TT>, a special encoding scheme was
used to translate hierarchical keywords name to MIDAS descriptor name
shorter than 16 characters. This is not needed anymore since long
descriptor name are supported from the <TT>96NOV</TT> release. For
backwards compatibility, a <TT>C</TT>-flag is avaliable in the <TT>INTAPE/FITS</TT> command providing the pre-96NOV name mapping.
<P>
The hierarchical structure provides a convenient and clear way to
separate information concerning different subsystems. The definition
of FITS keywords used by ESO for data acquisition can be found in the
<I>Data Interface Control Document</I> [<A
HREF="node126.html#dicb:96">7</A>]. This document
also gives a full definition of all hierarchical keywords in the
ESO-domain. FITS readers which do not know this convention should
save the <TT>HIERARCH</TT>-keywords as comments following standard FITS
rules.
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-09</I>
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